U.S. Pat. No. 6,274,857 (the 857 patent), incorporated herein by reference in its entirety, discloses a method of, and apparatus for, induction heat treatment of irregularly shaped workpieces such as selected components of a crankshaft. Using the reference numbers of, and referring to the figures in the 857 patent, a typical mating pair of bottom and top inductor segments (107) and (109), respectively, is illustrated in patent FIGS. 2(a), 2(b) and 2(c). The bottom inductor segment is connected to an alternating current (ac) power supply at power termination region (122) to form a single series loop active circuit from a pair of coil segments located around through opening (117a). Therefore the bottom inductor segment is also referred to as the active inductor segment. The corresponding top inductor segment (patent FIG. 2(b)) is a single turn closed loop coil, and can also be referred to as the passive inductor segment. At least one pair of coil lips, for example, coil lips (123a) and (123b) are formed around a partial opening, for example partial opening (121a), in at least one of the coil segments. A second pair of coil lips are formed in the top inductor segment, for example, around partial opening (121b) so that when the mating pair of bottom and top inductors segments are in the closed position, as shown in patent FIG. 2(c), a substantially closed inductor is formed around workpiece component (207) as shown, for example, in patent FIG. 6(a). Workpiece component (207) may be, for example, a crank pin on a crankshaft to which a piston connecting rod will be attached after metallurgical hardening. The pin may be attached at either end to counterweights (irregularly shaped adjacent workpiece components (206) and (208) in patent FIG. 6(b) or 6(c)). When the bottom and top inductor segments are in the closed position and alternating current is supplied to bottom segment inductor (107), magnetic flux concentrators, for example concentrators (103a) and (103b) in patent FIG. 2(c), are used to magnetically couple the flux created around the bottom inductor segment caused by current flow in the bottom (active) inductor segment so that a current flow having an instantaneous direction opposite to that in the bottom segment inductor is induced in the top (passive) inductor segment. In the closed position, dielectric material (410) separates the opposing facing surfaces of the bottom and top inductor segments as shown in patent FIG. 2(c). One or more side shields (137), as illustrated in patent FIG. 5(a) can be provided on one, or both, inner and outer sides of a coil segment around the arcuate coil region formed around a coil lip to serve as a magnetic flux concentrator for the workpiece component being heat-treated, and as a magnetic field shield for the workpiece components adjoining the heat-treated component. While the above describes a single turn, single coil for the bottom and top segment inductors, the 857 patent also discloses that a single coil, with two or more turns for either, or both, of the bottom and top inductor segments can be provided to harden relatively large individual workpiece components.
U.S. Pat. No. 6,859,125 (the 125 patent), incorporated herein by reference in its entirety, discloses an improvement of the 857 apparatus and method of induction heat treatment of irregularly shaped workpieces. Using the reference numbers of, and referring to the figures in the 125 patent, the bottom inductor segment (17) is connected to an alternating current power supply at power termination region (122a) and (122b) to form a double parallel loop active circuit from coil turns (16) and (18) as shown in patent FIG. 5. Current restricting slit (14) is used to form the double parallel loop active circuit and provide a more uniform current distribution across the parallel connected adjacent pair of coil segments. At least one of the parallel connected adjacent coil segment pairs has a partial opening, such as partial opening (21a) in coil segment (17a) in which an arcuate coil surface is formed. The arcuate coil surface can be formed into a pair of coil lips that are each separated by an orifice, as representatively shown in patent FIG. 5 as inner coil lips (23b), outer coil lips (23a), and orifice (31) in each of the adjacent coil segments. The coil lips are profiled to selectively compensate for the irregular mass of the irregularly-shaped component, for an opening on the surface of the substantially cylindrical component, or for selective heating of the fillet. Active inductor segment (17) may be mated with a single turn passive inductor segment as disclosed in the 125 patent. Alternatively active inductor segment (17) may be mated with two turn passive inductor segment (19) as shown in patent FIG. 6, or passive inductor segment (29) in patent FIG. 7, which is split into two electrically isolated coils (32) and (33) by cross sectional current restricting slit (30). When active inductor segment (17) is mated with one of the passive inductor segments, a workpiece can be inductively heated with the coil lip pairs as disclosed in the 125 patent.
The 125 and 857 patents generally address what is known as “band” heat treatment of workpiece components. For example where the selected workpiece component 207′ to be heat treated is the previously described crank pin, uniform heat treatment is generally required across the entire transverse surface region A′ of the pin as shown in FIG. 1(a) appended hereto, rather than fillet regions 207a′ and 207b′, which comprise the interface regions between workpiece component 207′ and adjacent irregularly-shaped components 206′ and 208′. Consequently, as illustrated in FIG. 1(a) and FIG. 1(b) appended hereto, the coil lips (bottom coil lip pair 123a′ and 123b′ partially shown in FIG. 1(a)) in the bottom and top inductor coil segments, 107′ and 109′, respectively, that surround workpiece component 207′, in combination, form a “band” of uniform induced heat around the entire transverse surface region A′ of the pin. FIG. 1(a) also shows representative side shields 137′, and FIG. 1(b) also shows representative dielectric 410′ that separates the facing surfaces of the bottom and top inductor coil segments.
The 857 patent discloses embodiments for heat treating the fillet regions B′, in combination with entire transverse surface region A′ of a workpiece component, by forming outward pointed tip regions on the coil lips 124a′ and 124b′ as shown, for example, in FIG. 2(a) appended hereto.
The 125 patent discloses positioning the pair of opposing coil lips in a pair of paralleled coil turns separated by a cross sectional current restricting slit so that they inductively heat only the fillet regions B′ between the selected workpiece component situated between a pair of coil segments and its adjoining workpiece component. As illustrated in FIG. 2(b) appended hereto, this is accomplished by making the between-coil-segments' cross sectional current restricting slit, S, relatively wide—in the range of 6 mm to 25 mm as taught by the 125 patent. First pair of coil lips 23a′ and 23b′ are on one side the slit, while second pair of coil lips 23c′ and 23d′ are on the opposing side of the slit. As disclosed in the 125 patent, the wide slit may be filled with a flux concentrator 138′ to further direct induction heating to the fillet regions B′.
The teachings of the 125 and 857 patents for heat treatment of only the fillet regions of a component workpiece, or selective heat treatment of the fillet regions and/or selective regions along the transverse width of the component workpiece, and metallurgical hardening of both the fillet and surface regions are somewhat limited. For example, where the component workpiece is a crankshaft pin or main journal having a narrow (for example, less than 30 mm wide) transverse bearing area (width), a “thumbnail” heating pattern C′ can result as shown in FIG. 3(a) and FIG. 3(b) appended hereto when utilizing the teachings of the 857 and 125 patents, respectively. The thumbnail heating pattern is generally not desirable due to several factors. First, such a heating pattern wastes energy since middle hardness depth must be appreciably deeper than necessary to achieve a satisfactory hardness depth towards the fillet regions. Secondly, such a heating pattern causes increased distortion of the heated component since increased heat absorption results in greater volumetric expansion of the component. Taking into consideration the shape of a complex workpiece, such as a crankshaft, the greater metal expansion leads to correspondingly greater shape distortion. In addition, the greater amount of metal heated above the phase transformation temperature results in a corresponding increase in lower temperature transformation structures such as martensite, lower bainite, and others, which in turn have different volumetric densities compared to the preheated metallurgy of the workpiece component. This also increases the shape/size distortion of the heat treated workpieces that has a “thumbnail” pattern. This thumbnail pattern can occur even though there is an absence of electrically conductive coil lips in regions 120′ between paired inner and outer active circuit coil lips 123a′ and 123b′ in FIG. 3(a), and regions 120″ between slit S, and paired coil lips 23a′/23b′ and 23c′/23d′ in FIG. 3(b). The thumbnail heating pattern can result from sufficient electromagnetic coupling between inner and outer coil lip pairs to create a sufficiently strong magnetic flux field in the center transverse region A′1 of the workpiece component shown in FIG. 3(a) and in the of center transverse regions A′2 of the workpiece component shown in FIG. 3(b). There is also reduced magnetic field strength in the opposing transverse end regions A′3 of the bearing surfaces due to the electromagnetic end effect of the inductor. Further there is an appreciable thermal heat sink effect due to the presence of the relatively cold (non-inductively heated) irregularly-shaped counterweights 206′ and 208′ located near both ends of the workpiece component 207; that is any inductive heating in end regions A′3 is conducted away from each transverse end region of the workpiece component and towards the adjacent irregularly shaped workpiece component.
One object of the present invention is to provide apparatus for, and method of, metallurgical heat treatment of cylindrical components of a complex workpiece, such as a crankshaft.
Another object of the present invention is to broadly control the induction hardening of cylindrical components of a complex workpiece across the transverse width and fillet areas of the cylindrical components.